One of the least charming but most persistent aspects of human nature is our capacity to hate people who are different. Racism, sexism, ageism, it seems like all the major social categories come with their own “-ism”, each fuelled by regrettable prejudice and bigotry.

Our tendency for groupness appears to be so strong there seems little more for psychology to teach us. It's not as if we need it proven that favouring our group over others is a common part of how people think – history provides all the examples we need. But one psychologist, Henri Tajfel, taught us something important. He showed exactly how little encouragement we need to treat people in a biased way because of the group they are in.

Any phenomenon like this in the real world comes entangled with a bunch of other, complicating phenomenon. When we see prejudice in the everyday world it is hard to separate out psychological biases from the effects of history, culture and even pragmatism (sometimes people from other groups really are out to get you).

As a social psychologist, Tajfel was interested in the essential conditions of group prejudice. He wanted to know what it took to turn the average fair-minded human into their prejudiced cousin.

He wanted to create a microscope for looking at how we think when we’re part of a group, even when that group has none of the history, culture or practical importance that groups normally do. To look at this, he devised what has become known as the “minimal group paradigm”

The minimal group paradigm works like this: participants in the experiment are divided into groups on some arbitrary basis. Maybe eye-colour, maybe what kind of paintings they like, or even by tossing a coin. It doesn't matter what the basis for group membership is, as long as everyone gets a group and knows what it is. After being told they are in a group, participants are divided up so that they are alone when they make a series of choices about how rewards will be shared among other people in the groups. From this point on, group membership is entirely abstract. Nobody else can be seen, and other group members are referred to by an anonymous number. Participants make choices such as "Member Number 74 (group A) to get 10 points and Member 44 (group B) to get 8 points", versus "Member Number 74 (group A) to get 2 points and Member 44 (group B) to get 6 points", where the numbers are points which translate into real money.

You won't be surprised to learn that participants show favouritism towards their own group when dividing the money. People in group A were more likely to choose the first option I gave above, rather than the second. What is more surprising is that people show some of this group favouritism even when it ends up costing them points – so people in group B sometimes choose the second option, or options like it, even though it provides fewer points than the first option. People tend to opt for the maximum total reward (as you'd expect from the fair-minded citizen), but they also show a tendency to maximise the difference between the groups (what you'd expect from the prejudiced cousin).

The effect may be small, but this is a situation where the groups have been plucked out of the air by the experimenters. Every participant knows which group he or she is in, but they also know that they weren't in this group before they started the experiment, that their assignment was arbitrary or completely random, and that the groups aren't going to exist in any meaningful way after the experiment. They also know that their choices won't directly affect them (they are explicitly told that they won't be given any choices to make about themselves). Even so, this situation is enough to evoke favouritism.

So, it seems we'll take the most minimal of signs as a cue to treat people differently according to which group they are in. Tajfel's work suggests that in-group bias is as fundamental to thinking as the act of categorisations itself. If we want to contribute to a fairer world we need to be perpetually on guard to avoid letting this instinct run away with itself.

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